Flood History Reconstruction in Frijoles Canyon Using Flood-Scarred Trees

McCord, V

Unexceptional summer storms triggered large floods on the Rito de los Frijoles in 1977 and 1978, shortly after the intense La Mesa Fire of 1977 burned this watershed. Large numbers of trees growing on or near the stream bank in Frijoles Canyon were scarred during these flood events. Such scars provide a record of the occurrence of past flood events which I used to reconstruct the flood history of the canyon. I compared the reconstructed flood history with existing fire history data to test the hypothesis that large floods have occurred following large-scale fires in the past. I sampled 36 scarred trees at several localities along the Rito de los Frijoles. The flood scars were crossdated using reference chronologies I developed from scarred and unscarred trees in the canyon. I surveyed channel cross-sections and the top heights of the flood scars above the stream level, and then reconstructed paleoflood discharges using the slope-area method. The riparian tree-ring chronology extends to the early 1600s, represented by at least five trees back to the mid-1700s. The scar dates range from 1773 to 1985, with most of the scar dates falling in 1977 and 1978. Floods were also identified for 1866, 1904, 1951, and 1972. Other than 1977, only the 1773 flood(?) scar matches a major fire year in the local fire scar record - this suggests that fire intensity, rather fire extensiveness, is the major factor leading to post-fire flooding in this watershed. Flood discharge estimates reconstructed from flood scars correspond well with the USGS estimates for the historic floods of 1977 and 1978. The flood scar evidence in Frijoles Canyon indicates that there have been at least four floods comparable to the 1978 flood in the last two centuries, and at least seven floods as large as the flood of 1977 during that time.